17 Sep
2016
17 Sep
'16
11:38 a.m.
On Sep 17, 2016, at 10:10 AM, Chuck Guzis <cclist
at sydex.com> wrote:
On 09/17/2016 09:23 AM, Eric Smith wrote:
Why? What does the width of the ALU have to do with the ?bitness? of the
architecture? If the programmer?s view is 8-bits (or 16 or 23, or ??),
what does it matter (other than performance) what the width of the internal
data paths or ALU are?
It?s interesting from an implementation point of view but not really
anything else.
In a previous email, I mentioned the IBM 360 as an example of a 32-bit machine
(architecture) that had 8, 16 and 32 bit internal data paths and I don?t think
anyone would suggest that the 360 models that did not have 32-bit data paths
wasn?t a 32-bit ?machine?.
The same could be said for the PDP-8/s. That?s a bit serial machine but it
is a member of the PDP-8 family. Would you call it a 1-bit machine or a 12-bit
machine?
That?s why (in my previous email) I made the distinction between architecture
and implementation. The reference ?machine? (which I?ve intentionally used here)
is somewhat ambiguous and I tend to use architecture or implementation when I
want to be specific.
TTFN - Guy
I don't know what the width of the TMS9900
ALU is, but I'm pretty
sure it's not bit-serial, as an add instruction only takes 14 clock
cycles, including four memory cycles. I'd be very surprised if the
ALU isn't either 8 or 16 bits, though 4 might be possible.
Possibly someone is confused by the bit-serial "CRU' I/O space, but
that is unrelated to the ALU width.
Could very well be--I'm just going by other's appraisals of the
architecture.
But there were some "8 bit" MPUs with bit-serial ALUs, so the question
is still valid.